ClassPool classPool = ClassPool.getDefault(); CtClass ctClass = classPool.get("com.example.MyClass"); CtMethod method = ctClass.getDeclaredMethod("myMethod"); method.insertBefore("{ System.out.println(\"Before method\"); }"); method.insertAfter("{ System.out.println(\"After method\"); }"); CtMethod newMethod = CtNewMethod.make("public void myNewMethod() { System.out.println(\"New method\"); }", ctClass); ctClass.addMethod(newMethod); ctClass.writeFile(); ClassPool classPool = ClassPool.getDefault(); CtClass ctClass = classPool.makeClass("com.example.GeneratedClass"); CtField field = new CtField(CtClass.doubleType, "myField", ctClass); ctClass.addField(field); CtMethod method = CtNewMethod.make("public void myMethod() { System.out.println(\"Generated method\"); }", ctClass); ctClass.addMethod(method); ctClass.writeFile();